Determine whether the marked reduction in longevity of adult female Xyleborus ferrugineus, when given the non delta 7-sterol, cholesterol, as their sole dietary sterol, is attributable chiefly to a correspondingly earlier depletion of essential delta 7-steroids or metabolites. Specific aims would be (1) to quantify the amount and major distribution in organs of 3H-steroids including ecdysteroids accumulated from the required diet by newly metamorphosed virgin adult females, and then correlate changes in this in vivo 3H-steroidal amount with subsequent changes in other given observable parameters of aging (e.g., locomotory vigor, reproductive capability, and death); (2) to compare relative rates of depletion of this essential initial 3H-steroid pool (meter) in the new adult female when the insects are subsequently aged on diet containing a non delta 7-sterol, cholesterol, versus a delta 7-sterol, 7-dehydrocholesterol, as the sole dietary sterol; and (3) to establish further the ultrastructural parameters in critical tissues which accompany the aging as monitored through 3H-steroids and the other mentioned parameters. Insects would be germ-free and raised and aged continuously on chemically defined diet. TEM would be used to analyze ultrastructure of tissues. 3H-steroids would be quantified in various aged insects using scintillation counting of radiolabel of extracted steroids from specific tissues. Autoradiography with light and TEM would also be utilized to estimate amount and location of 3H-steroids.